The work aims to study the thermal insulation performance of EPP insulating box with different structure sizes in the process of heat transfer, and the phase change process and melting time of the phase change material in the EPP insulating box. The finite element software was used to construct the heat transfer model of EPP insulating box and the phase change heat transfer model of phase change materials. The time in which the phase change material was completely melted was taken as the effective holding time, and the length of the effective holding time was used to evaluate the insulation performance of the insulating box. In the heat transfer process, when the inner volume, insulating box thickness and lid thickness were the same, the smaller the geometric mean of the internal and external surface areas, the longer the holding time. When the length-width ratio was between 1.8 and 2.0, the geometric mean of internal and external surface areas was the smallest. In the process of structure design of insulating box, the appropriately controlled length-width ratio can reduce the geometric mean of the internal and external surface areas, thus effectively improving the insulation performance of the insulating box.